Storage tank bottoms, particularly those in the oil and petroleum industries, are continuously threatened by corrosive species and moisture present in the environment. When located near a body of salt water, the exposure to saline heightens this problem. Storage tanks and base supports are exposed to exceptionally high loads. For safety and environmental reasons, it is imperative that these base supports and tank bottoms remain safe, secure, and intact, and unimpaired by corrosion.
In the past, minor leakage from a storage tank was an acceptable practice. Current environmental regulations and accepted practices, however, make leakage a matter of great concern. Vast amounts of groundwater can be contaminated, and cleanup costs can be prohibitively expensive. Also, leaking tanks can make a site not saleable on the open market, and may jeopardize the positive public image of a company.
Storage tank bottoms are commonly protected from corrosion using cathodic protection (CP). One type of cathodic protection common in the art is the use of a sacrificial anode. If two dissimilar metals (electrodes) such as a zinc sacrificial anode and a steel storage tank, are immersed in a conductive liquid (electrolyte) and a voltmeter is placed between them, an electrical potential difference between these electrodes will be measured. In this particular cell, the less noble metal, zinc, is called the sacrificial anode, and the more noble metal, steel, is called the cathode. The current causes electrochemical reactions to take place around the anode as well as around the cathode. The anode (zinc) slowly dissolves in the electrolyte, such as water, while protecting the cathode from such corrosion.
Another type of cathodic protection commonly employed in the art is that of an impressed current. In this case, a current is supplied to the tank from an external direct current source. The amount of current provided via this method can be much greater than that obtained using a sacrificial anode.
In both types of cathodic protection the current is generally delivered to the storage tank via the base support.
However, problems can occur where the storage tank is not in complete contact with the base support. This can occur, for example, when the storage tank is being filled and emptied, causing the bottom of the storage tank to buckle slightly, leaving air gaps. Other times, a portion of the base support may erode. In such cases, electrical conductivity is lost between the storage tank and base support, compromising the corrosion resistance provided by cathodic protection.
Newer storage tanks are designed with secondary containment such as double bottoms that detect leaks and contain product migration in the event of a leak. Even with such systems in place, corrosion protection must still be addressed to minimize the occurrence of leaks.
The problems that arise when there is not complete contact between the storage tank and base support can be controlled with the proper use of vapor phase corrosion inhibitors (VpCI). Various compositions of these inhibitors are known to provide corrosion protection under wet conditions, corrosive environments and in void spaces, as experienced in a storage tank and base support arrangement. Unfortunately, exposure to cathodic protection can adversely affect the performance of many VpCI compositions. For example, some VpCI compositions lose effectiveness or even enhance corrosion when exposed to a cathodic environment.
It would, therefore, be advantageous to provide one or more VpCI compositions that are effective both independently and in the presence of cathodic protection.